Lift-off correction of printing by conventional typewriters is now a standard option. To achieve such correction, the cohesion of the ink in a printed character must be greater than the affinity of the ink in the character for the paper or other surface upon which it is printed. The ink is formulated so that the adhesion is one of surface adhesion between the ink and the paper rather than a viscous penetration of the paper fibers or wetting of the paper fibers with the ink layer. With such ink as the printing material, correction of erroneously typed characters is accomplished by adhesive removal from the surface of the image sheet or paper, using a piece of material having an adhesive surface, where the adhesive surface is impacted onto the erroneously typed letter. This adheres the adhesive surface of the correction material to the character, and the adhesive element is pulled from the paper, thereby pulling the erroneously typed character bodily with it. This now-standard lift-off correction with conventional typewriters is illustrated by U.S. Pat. Nos. 3,825,437 to Blair and 3,825,470 to Elbert et al. Numerous other prior art to the same general effect might be cited, but such additional teachings are considered cumulative at most because they do not involve thermal printing.
Conventionally, the character erroneously typed is the character once again impacted during lift off erasure. This form of impact minimizes adhesion to the paper surrounding and in internal uninked parts of the character. Abrasion and other marking of the paper is thereby minimized.
Thermal printing of the kind here involved is known and described in the prior art, but is presently very much less common than conventional impact typing. U.S. Pat. No. 3,744,611 to Montanari is illustrative the basic printing system and U.S. Pat. No. 4,103,066 to Brooks et al. describes a ribbon with a polycarbonate resistive layer for thermal printing. Neither of these patents mentions correction of erroneously printed characters. IBM Technical Disclosure Bulletin, Vol. 23, No. 5, (October 1980) page 2012, "Electrothermal Ribbon Path," by S. L. Applegate et al. discloses thermal printing in which the ribbon is directed away from print area while still warm so as to minimize adhesion to the ribbon after printing found to occur with cooling.
A non-tacky roll is easier to feed and otherwise handle within the typewriter, and reduction and elimination of tack in a lift-off correction ribbon except during the correction step is now a commonly recognized design objective. U.S. Pat. No. 3,855,448 to Hanagata et al. and IBM Technical Disclosure Bulletin, Vol. 19, No. 2, (July 1978), page 672, "Delayed Tack Ribbon for Laser Transfer and Other Printing," by C. A. Bruce et al., both are to thermal printing and both describe their transfer layer as an adhesive material which is non-adhesive until the temperature is raised during printing. Neither have any mention of lift-off correction. In U.S. Pat. No. 4,093,772 to Taylor et al. and U.S. Pat. No. 3,924,728 to Brown et al. a lift-off correction tape is part of the typewriter ribbon and is said to be non-tacky during feeding. The coating in these patents is said to be not sticky to touch and not adherent to itself, but to become sticky in response to pressure, specifically the pressure of impact typing. U.S. Pat. No. 3,998,314 to Barouh et al. is to the same general effect, but describes the lift-off layer only as impact compressible.
Typically, in the prior art the lift-off correction tape is fed by mechanisms separated from the imaging ribbon feed mechanisms. Desirable aspects of a combined or single ribbon feed are recognized. Thus, the above-mentioned U.S. Pat. Nos. 4,093,772 and 3,924,728 show a dual ribbon with lengthwise strips, one of marking material and one of lift-off correction material. This is said to be a conventional split correction ribbon with a lift-off coating rather than a masking coating. The normally non-tacky nature of the lift-off strip is said to make possible the feeding and handling of the dual ribbon by a single mechanism in the typewriter. U.S. Pat. No. 4,034,843 to Newman et al. similarly discloses a split, lift-off correction-imaging ribbon for impact typing, with emphasis on techniques of joining the two strips.